This invention relates to vacuum breakers, and more particularly to vacuum breakers for use on movable tanks for allowing atmospheric air to enter the tank to relieve a reduced pressure condition in the tank.
Tanks for carrying normally liquid products on the highway, by rail and on water are typically designed to withstand substantial positive internal pressure relative to atmospheric pressure. The resistance of such tanks to negtive or reduced internal pressure relative to atmospheric pressure, however, is typically much less. These tanks are therefore subject to considerable damage or even complete destruction due to implosion in the event that pressure in the tank falls significantly below atmospheric pressure, an event referred to herein as a reduced pressure condition in the tank.
A reduced pressure condition may result from any of a variety of circumstances. The tank may be loaded with a hot or warm product, and as the product cools, its vapor pressure decreases, thereby reducing pressure in the tank. Shifting of the product in the tank as the tank moves may create localized zones of reduced pressure in the tank. The tank may be cleaned with hot liquids or vapors, and if the tank is left closed after cleaning, pressure in the tank may fall substantially below atmospheric pressure when the tank cools. Changes in ambient temperature or pressure (e.g., due to a rapid change in altitude) may cause a reduced pressure condition in the tank. Inadvertent failure of those handling the tank to open the tank vents during unloading of the tank may also cause such a condition.
If the reduced pressure in the tank is great enough, it may cause implosion of the tank. This is both dangerous and costly. A full size highway tank trailer now costs approximately $50,000 new. The cost of repairing such a tank which has been extensively damaged by implosion may be $18,000 to $28,000.
Vacuum breakers for mounting on top of movable tanks for admitting atmospheric air to relieve a reduced pressure condition in the tank are known. Typically, these known vacuum breakers include a plate or other closure which is spring-biased against a closure seat. In theory the spring keeps the vacuum breaker closed until the difference between atmospheric pressure and the pressure in the tank is great enough to overcome the spring force, thereby opening the vacuum breaker and admitting air to the tank. In practice, however, these prior vacuum breakers have often been unsatisfactory.
Vacuum breakers on moving tanks are subject to considerable vibration as the tank moves along the highway, railway, or on rough water. Unless the spring is very strong (which may prevent the vacuum breaker from opening as required in response to relatively small reduced pressure conditions in the tank), the closure will be constantly opening as a result of this vibration. This so-called "road flutter" of the vacuum breaker fatigues and weakens the spring so that the working life of the vacuum breaker is considerably shortened. Road flutter is also environmentally undesirable in that it may allow product and product vapor to escape from the tank. Many of the products carried in such tanks (e.g., strong acids, bases, volatile petroleum products and the like) are extremely hazardous, corrosive and environmentally unsafe. In addition to its environmental unacceptability, the escape of product and product vapor from the tank may interfere with operation of the vacuum breaker, e.g., as a result of product residues accumulating in the vacuum breaker. Such residues may either leave the vacuum breaker permanently open (permiting continued escape of product or product vapor), or effectively sealed closed (so that the vacuum breaker can no longer perform its function and the tank is again subject to implosion).
In view of the foregoing, it is an object of this invention to provide improved vacuum breakers for movable tanks.
It is a more particular object of this invention to provide vacuum breakers for movable tanks which are highly resistant to opening due to vibration of the tank (road flutter), but which open readily in response to relatively small reduced pressure conditions in the tank.